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A Virus-Infecting Virus

Analyzing the waters of a Parisian cooling tower for new strains of a so-called “giant virus,” a team of scientists discovered more than it hoped for. Not only did it find a new giant specimen, it also identified a hitherto unknown type of infectious agent–a virus-infecting virus.A first strain of this giant virus, the Acanthamoeba polyphaga mimivirus (APMV) had already been discovered in 2003, inside the waters of a UK water-cooling tower by the team of Didier Raoult from URMITE.1 This “mimivirus,” which mostly affects the amoeba, is so big that it is visible under the optical microscope. With over 900 protein-coding genes, it is at least three times bigger than other known viruses, and even bigger than some bacteria. But now these same scientists have identified an even bigger specimen, the “mamavirus,” which revealed, under electron microscopy, an unfamiliar travel companion–a 50 nm virus the team called “Sputnik.” In their study,2recently published in Nature, the researchers describe that Sputnik is unable to multiply in an amoeba on its own, but does grow rapidly when it can penetrate APMV's virus factory, where new viral particles are made. The hijacking of the factory's machinery results in the production of non-viable forms of APMV. Sputnik is therefore harmful to its host virus. “This shows that there are viruses that attack other viruses and are capable of diminishing their virulence. Maybe one day we could be fighting viruses with other viruses,” says Didier Raoult.The researchers classified Sputnik as a “virophage,” because of its functional analogy with bacteriophages, which similarly infect and weaken bacteria. An analysis of Sputnik's 21 genes by Raoult's team, along with French and American colleagues,3 revealed that the viral genome contains APMV genes, but also a gene from an archaeal virus and two genes very similar to those of bacteriophages. This finding suggests the virophage could be involved in genetic transfers between giant viruses, another analogy with bacteriophages.